Improve SEVIRI calibration error message

[sfinkens]

Improve error message if SEVIRI calibration coefficients are missing.

• Closes NotImplementedError: Unknown calibration type #3281
• Tests added

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.29%. Comparing base (b02f62d) to head (79b3e5f).
⚠️ Report is 82 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3293      +/-   ##
==========================================
- Coverage   96.30%   96.29%   -0.02%     
==========================================
  Files         463      463              
  Lines       58307    58873     +566     
==========================================
+ Hits        56155    56691     +536     
- Misses       2152     2182      +30     

Flag	Coverage Δ
behaviourtests	3.60% <0.00%> (-0.04%)	⬇️
unittests	96.38% <100.00%> (-0.02%)	⬇️

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[mraspaud]

LGTM, thanks for taking the time to improve the test code also!

[coveralls]

Pull Request Test Coverage Report for Build 19292873099

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 96.372%

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Totals
Change from base Build 19291198902:	0.0%
Covered Lines:	56400
Relevant Lines:	58523

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💛 - Coveralls